The abbreviation of Traveller Mailing List, this is the Mailing List that discusses everything related to the Traveller role playing game and other general Science Fiction topics, as long as there is relevance to Traveller. To quote the TML-FAQ:
The Traveller Mailing List (TML) is open to discussions on all aspects of Traveller. It exists as a means for the Traveller Player/GM to exchange ideas and discuss the various aspects of the Traveller System, and its universe. This includes discussions on GURPS Traveller, Marc Miller's Traveller, Traveller The New Era, MegaTraveller and Classic Traveller. This also includes the background.
Be warned that this list is a high volume mailing list, usually averaging 80-100 mails per day, but sometimes swelling up to thrice that volume, so be warned. There is also a digest version that reduces the amount of individual mails received.
The TML split into the X-Boat List and the TML upon the release of Traveller: The New Era, when differences between those who refused to acknowledge the new system and those who accepted it's premises became unreconcilable. Those who continued to play Classic or Mega subscribed to X-Boat, while the others remained on the TML, although there was a lot of crossmembership in those days. When GDW folded, the two lists rejoinded, and X-Boat was abandoned, the rift between both groups slowly vanishing.
One might also want to note that the TML is (and has been for a long time) the prime pool of new talent for Traveller. Many writers of GURPS Traveller (and the better T4 sourcebooks first wrote on the TML and are usually still active and present. Many of the original designers, such as Loren Wiseman and Marc W. Miller are also actively involved in discussions, or are at least reading them. The TML is also an unmissable mine of ideas, adventures, campaigns, background on the universe, thoughts on specific rules or events and many other things to help you play.
The TML-FAQ can be found using the link below. Study it before you post!
http://www.downport.com/traveller/tml/tml-faq.html
This FAQ includes subscription information, etiquette, topics done to death (such as the dreaded female aslan pirates onboard their modified near-C rocks carrying a load of Jump torpedoes to any Virus ridden Feudal Technocracy), as well as other things one should know before entering this snakepit. Trolls are not tolerated and are usually dealt with swiftly. Newbies are usually tasked to write a Newbie-essay an one of any given amount of topics related to Traveller, suggested by the veteran members. These are not mandatory, but usually they are written.
About every 6 months the TML usually explodes into an ugly flamewar, usually Off-Topic for about a week or so, before everything settles down again, and everybody is friends again. Just don't say I didn't warn you...